A conventional drive-in theater employs a centralized audio system that reads the soundtrack of a film and produces a carrierless amplitude modulated audio signal. That signal is distributed by wire to speakers at the individual parking spaces. It is the speakers that often account for a low quality sound and result in a continuing high-cost maintenance and replacement problem. They are often stolen, vandalized, accidently damaged, or adversely affected by weather conditions.
An alternative to the conventional deployment of speakers is the substitution for the entire conventional audio system of a relatively low power radio transmitter. This system broadcasts at a carrier frequency that can be received by a conventional AM automobile radio, thus eliminating the speakers completely. In other words, each patron simply listens to the audio portion of the film on the radio of his own automobile. An elaborate transmitting antenna system is often incorporated in such systems to make contact with the radio antenna of each automobile. One such system is described in U.S. Pat. No. 4,047,109 to Sekiguchi.
A number of marked disadvantages are associated with the use of known radio transmission systems in drive-in theaters. One problem is that this system, with its large radiating antenna, may interfere with radio reception in the surrounding area. It is often difficult or impossible to assess the extent to which such interference will occur until the system has been built and installed.
Another problem arises from the fact that some automobiles are not equipped with working radios or have such weak batteries that their radios cannot be used in this way. An individual speaker cannot readily be provided for such automobiles since a speaker will not respond properly to the available modulated carrier signal.
Still another disadvantage of the substitution of a control radio transmitter for an existing drive-in sound system of conventional construction is that essentially all components of the original system must be scraped and replaced. Moreover, it is usually impractical to replace only a portion of the system so that the operator of the theater has little opportunity to experiment with the system or phase it in over a period of time as his customers become accustomed to it.
It is a principle objective of the present invention to provide an improved audio system for drive-in theaters that employs automobile radios as a part of the system, but also incorporates a large part of the existing system. Another objective is to provide such a system that minimizes or eliminates interference with radio reception in the surrounding area. Still another objective is to provide such a system that is compatible with the use of conventional speakers and, therefore, permits partial as well as full conversion.